Meade MA 12MM Operating Instructions

Introduction: The Meade MA 12mm Astrometric Eyepiece

(1.25” O.D.) functions as a multiple measuring device for determining angular diameters and position angles of celestial objects. It is also an excellent guiding eyepiece, used to monitor a guide star through an off-axis guider or guide telescope during long-exposure astrophotography.

The precision-etched glass reticle is illuminated by a continuously variable-brightness red LED (Light Emitting Diode), that can be dimmed to allow observations of very faint objects. The eyepiece can be focused to match the user’s eye diopter so that the scales of the reticle can be examined in sharp focus.

Focusing the Eyepiece: To focus the reticle of the Astrometric Eyepiece, turn the Eyelens Focuser (1, Fig. 1) of the eyepiece clockwise or counterclockwise until the reticle scales are seen at their sharpest.

Using the LED Illuminator: To turn on the illuminator, rotate the illuminator control knob (3, Fig. 1) past the click stop. Illumination brightness control is varied by further turning the control knob. Turning clockwise increases the brightness of the LED, while counterclockwise decreases the brightness. To turn the illuminator off, turn the control knob all the way counterclockwise until it is turned to the click stop.

Astrometric Reticle Scales: There are 4 reticle scales etched into the glass reticle (Fig. 2) of the eyepiece that correspond to the table below:

Scale Type Description

1 Linear Diameter 0 to 50 Units; Segmented

Scale in Single Units with

Circular Center Position

2 Semicircular Position 0 to 90 Degrees with

Angle Scale Circular Center Position

3 360° Position Angle Segmented in Units of 5°

Scale

4 Double Crossline/ Four Concentric Circles

Concentric Circle with Each Inner Circle

Guiding Scale Representing .5 Width,

.25 Width, and .10 Width

of the Outer Circle

Dimension, Bisected

with a Double Crossline

Determining Image Scale: An accurate determination of the image scale for a given telescope can be accomplished by making visual timings of a star as it drifts through the Astrometric Eyepiece. To make the measurement follow these steps:

1. Set up the telescope and insert the Astrometric Eyepiece with the LED Illuminator turned on.

2. Check the collimation of the telescope (refer to the telescope’s instruction manual). A collimated telescope produces the best image possible.

3. Bring the telescope to a sharp focus.

4. Aim the telescope at or very near (within +/- 5°) the zenith (perpendicular to the ground).

5. Turn off the telescope motor drive.

6. Rotate the Astrometric Eyepiece so that a star drifts along (

i.e.

, parallel to) the Linear Diameter Scale (1, Fig. 2) of

the reticle.

7. Take three timings (in seconds) of a star as it crosses the Linear Diameter Scale. Begin the timing as the star crosses one edge of the scale and end the timing as the star crosses the other edge of the scale.

8. Record the timings and average them.

With the timings recorded and averaged, an accurate determination of the angular diameter of the sky that the Linear Diameter Scale represents can be calculated. Stars at or near the zenith move across the sky at 15 arc seconds per second (sidereal rate). To determine the true angular diameter in arc seconds, multiply the number of seconds of the averaged timing of the scale by 15. This number represents the entire length of the scale in arc seconds. To know the number of arc seconds represented by each of the segments of the scale, divide by 50. To know the value in arc minutes, divide arc seconds by 60. To know the value in degrees, divide arc seconds by 3600.

Example:

If the star takes 10 seconds to cross the entire length of the scale:

seconds X 15 = angular diameter in arc-seconds

10 X 15 = 150 arc-secs angular diameter

arc-secs ÷ 60 = angular diameter in arc-minutes

150 ÷ 60 = 2.5 arc-mins angular diameter

arc-secs ÷ 3600 = field of view in degrees

150 ÷ 3600 = 0.041° angular diameter

Use the above techniques and formulas for making measurements of any of the scales on the reticle of the Astrometric Eyepiece. Repeat these steps if the Eyepiece is used on a different setup (

e.g.

, used with a different diagonal,

a Barlow lens, etc.).

Knowing the measurements of the scales of the reticle, the observer can take measurements to determine apparent diameters or angular separations of celestial objects (

e.g.

, planets, deep-sky objects, craters on the Moon, double stars, etc.). Other measurements can be made, such as the periodic error of the telescope’s drive system.

Operating Instructions

Meade®MA 12mm Astrometric Eyepiece

Fig. 1: MA 12mm Astrometric Eyepiece. (1) Eyelens Focuser; (2) Red LED Illuminator with Continuously Variable Brightness Control; (3) Brightness Control Knob.

MEADE MA

ASTROME

ILLUMINATED

Determining Position Angle: Position angle (PA) in astronomical terms is the direction in the sky of one celestial body with respect to another, measured from 0° to 360° in an easterly direction from north. Position angles are used by astronomers, for example, to give the position of a tail of a comet with respect to the nucleus of the comet, or to measure the position angle one component of a multiple star to another.

To make a position angle measurement, calibrate the orientation of the 360° Position Angle Scale (3, Fig. 2) along the east-west line of the sky by following these steps:

1. Set up the telescope and insert the Astrometric Eyepiece with the LED illuminator turned on.

2. Check the collimation of the telescope (refer to the telescope’s instruction manual). A collimated telescope produces the best image possible.

3. Bring the telescope to a sharp focus.

4. Aim the telescope at or very near (within +/- 5°) the zenith (perpendicular to the ground).

5. Turn off the drive of the telescope.

6. Release the set-screw of the eyepiece holder and rotate the Astrometric Eyepiece within the eyepiece holder so that a star drifts across the 90° mark of the Position Angle Scale and then exits across the 270° mark.

7. Tighten the set-screw of the eyepiece holder to lock the position.

Once the scale is successfully calibrated along the east-west line of the sky, follow these steps to measure the position angle of a celestial object:

1. Place the reference object (

e.g.

, primary component of a comet nucleus) in the circular mark at the center of the Linear Diameter Scale (1, Fig 2.).

2. Find the object that the position angle is measured for (

e.g.

, discernable edge or center line of a comet’s jet or tail

that extends across the eyepiece field of view).

3. Read the position angle.

For example, if the nucleus of a comet is placed in the center of the Linear Diameter Scale, and the center of the tail (the object the PA measures for) is in-line with the first segment after 300 on the Position Angle Scale, then the PA of the center of the comet’s tail is 305°.

Important Note: The Position Angle Scale reads clockwise from 0° to 360° as used through mirror-lens Cassegrain telescopes (

e.g.

, Schmidt-Cassegrain, Maksutov-Cassegrain), or refractors where a diagonal mirror/prism is being used. If the Astrometric Eyepiece is being used without a diagonal prism or through a Newtonian reflecting telescope, the position angle reading should be reversed (

e.g.

, 90° translates to 270°)

If the PA of a double-star, or other closely spaced objects is to be measured, use the Semicircular Position Angle Scale (2, Fig. 2). To make a measurement on this scale follow the same steps outlined above, but with these important differences:

• The Semicircular Position Angle Scale reads from 0° to

90° clockwise and counterclockwise. The clockwise direction is for measurements being made through mirrorlens Cassegrain telescopes (

e.g.

, Schmidt-Cassegrain, Maksutov-Cassegrain), or refractors where a diagonal mirror/prism is being used. Translate the 90° mark on the opposite side of the scale as 270°.

• The counterclockwise direction is for a Newtonian reflecting telescope or a telescope without a diagonal mirror/prism. In this case the 90° mark should be on the “left” side of the scale. Translate the 90° mark on the opposite side of the scale to 270°.

270

240

210

180

150

120

330

300

0 1020304050

Fig. 2: MA 12mm Astrometric Eyepiece Reticle. (1) Linear Diameter Scale, (2) Semicircular Position Angle Scale; (3) 360° Position Angle Scale; (4) Double Crossline/Concentric Circle Guiding Scale.

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